1. Field of the Invention
The present invention relates to a thin film transformer having a spiral thin film coil and more particularly to a technology for forming a coil consisting of a conductive material.
2. Description of the Prior Art
Thin film transformers formed on semiconductor substrates consisting of silicon or the like are known. Such transformers can be small in size because they are fabricated by a thin film development technology. They are among the electronic devices for forming semiconductor integrated devices. A conductive wiring pattern made of a conductive material or semiconductor is used for forming coils in thin film transformers. The shape of the coils is selected to be spiral in order to obtain a large Q-value (Q=.omega.L/R where .omega. is angular frequency, L is mutual inductance and R is the resistance of the coil). An example of a thin film transformer with a spiral structure is shown in FIGS. 1A and 1B. FIG. 1A is a plan view showing the structure of a conventional thin film transformer, and FIG. 1B is a cross-sectional view taken along the line I--I in FIG. 1A. As shown in FIGS. 1A and 1B, a thin film transformer 130, which is formed on a substrate 131, includes a silicon dioxide layer 132a, a primary coil 133, a silicon dioxide layer 132b, a secondary coil 134, and a silicon dioxide layer 132c superimposed on the substrate 131 in this order. The hatched region in FIG. 1A indicates a region in which the primary coil 133 and the secondary coil 134 overlap when viewed from above or in projection. The thin film transformer 130 is formed as follows. First, the silicon dioxide layer 132a is deposited on the surface of the substrate 131 to a thickness of from 0.1 to 2 .mu.m. A highly conductive metallic material such as aluminum is deposited on the upper surface of the silicon dioxide layer 132a to a thickness of from 1 to 3 .mu.m by a sputtering method or a vacuum deposition method to form a metallic film. Next, the metallic film thus formed is processed by lithography and etching in order to transfer spiral patterns to produce a metallic line having a width of from 50 to 200 .mu.m and having a wiring spacing or pitch of from 50 to 200 .mu.m. The metallic line forms a coil 133 and has a spiral pattern, with a plurality of corners at which two adjacent metallic line segments merge with each other. After the further silicon dioxide layer 132b is formed to a thickness of from 0.1 to 2 .mu.m on the primary coil layer 133, the secondary coil layer 134 is formed on the silicon dioxide layer 132b to a thickness of from 1 to 3 .mu.m in a manner similar to the primary coil layer 133. Then, the silicon dioxide layer 132c is formed to a thickness of from 1 to 2 .mu.m on the surface of the primary coil 134 layer. In order to make both ends 135a and 135b of the primary coil 133, and both ends 136a and 136b of the secondary coil 134, exposed for electrical connections, the silicon oxide layers 132b and 132c above the end terminals 135a, 135b, 136a, and 136b of the primary coil 133 and the secondary coil 134 are each partially removed by lithography and etching, and finally the thin film transformer 130 is completed. In the thin film transformer 130, the numbers of turns of the primary coil 133 and the secondary coil 134 are each 4, and the secondary coil 134 has the same pattern as the primary coil 133 and is positioned in the same area as that occupied by the primary coil 133. In other words, their projected areas overlap completely except for the terminals.
In a thin film transformer formed as described above, a modification of the quantity of current running from the end 135a to the end 135b of the primary coil 133 results in a change in the magnetic field generated around the primary coil 133, and an electric potential difference appears between the ends 136a and 136b of the secondary coil 134 to generate electromotive force. The induced electromotive force (induced current) generated in the secondary coil 134 is proportional to the number of turns of the secondary coil 134. The larger the number of turns of the primary coil 133, the higher the intensity of magnetic field generated by the primary coil 133, which leads to generating a larger induced electromotive force in the secondary coil. Thus, in the thin film transformer 130 which produces electromotive force by means of mutual inductance between the coils 133, 134, the larger the numbers of turns of the primary coils and the secondary coils, the higher the intensity of the magnetic field generated by each of the coils so that the inductance between the coils increases, and also the coupling coefficient becomes larger, resulting in that the efficiency of energy conversion from the primary coil 133 to the secondary coil 134 can be increased.
However, a thin film transformer formed as described above suffers from various problems. For example, if the numbers of turns of the primary coil 133 and the secondary coil 134 is increased, the overall area of the thin film transformer 130 becomes larger, which hinders the fabrication of small-sized transformers. In addition, increasing the numbers of turns of the coils leads directly to an increase in the length of the coils. A thin film conductor has a resistance which is generally much higher than the resistance of a wire. Hence, a problem would arise in that the energy loss due to the increased resistance of thin film coils when their length is increased could cause a reduction of Q-values, which serves as an index of energy conversion efficiency.
Thus, in the conventional thin film transformer 130, an increase in the number of turns of the coils for increasing the energy conversion efficiency and a reduction in the size of the coils have a trading-off relationship, and there is a possibility that increasing the number of turns may cause reduction in the energy conversion efficiency.